A vehicle-use communication system is in practical use in which locking or unlocking of a vehicle door is achieved without the use of a mechanical key. Specifically, such systems in practical use include: a keyless entry system in which locking or unlocking of a vehicle door is achieved by wireless remote operation by using a portable device owned by a user; and a smart entry (registered trademark) system in which a vehicle door is unlocked when a user having a portable device merely approaches the vehicle or merely grips the door handle.
Further, a vehicle-use communication system is also in practical use in which engine start in a vehicle is achieved without the use of a mechanical key. Specifically, a smart start system is in practical use in which the engine is started when a user having a portable device merely pushes an engine start button.
In the communication systems described above, the in-vehicle device performs communication with the portable device by using radio signals so as to perform authentication and, after that, perform control concerning the given operation of unlocking, locking, engine start, or the like. However, for the purpose of preventing unauthorized operation, before the operation is executed, it is checked whether the portable device is located at a given position. As methods of checking the position of the portable device, methods described in Japanese Patent No. 4483236, Japanese Patent No. 4673234 and Japanese Patent No. 4673251 or the like are known.
Japanese Patent No. 4483236 discloses a position detection method in which a signal is transmitted from an antenna for vehicle cabin inside and an antenna for vehicle cabin outside to the inside and the outside of the vehicle cabin and then the position of the portable device is determined on the basis of which signal triggers a response from the portable device.
Japanese Patent No. 4673234 discloses a keyless entry apparatus in which a portable device receives signals transmitted from a plurality of antennas provided on a vehicle and then the position of the portable device is determined on the basis of the received signal strengths of the signals received by the portable device. The in-vehicle device according to Japanese Patent No. 4673234 stores, in advance, data required for determination of the position of the portable device.
Specifically, the portable device is arranged at a plurality of locations along the inner side of the vehicle cabin inside-outside boundary and then the portable device receives signals for received signal strength measurement (for portable device position determination) transmitted from the plurality of transmitting antennas connected to the in-vehicle device so as to measures the received signal strengths of the signals. Similarly, the portable device is arranged at a plurality of locations along the outer side of the vehicle cabin inside-outside boundary and then the portable device receives signals for received signal strength measurement transmitted from the plurality of transmitting antennas connected to the in-vehicle device so as to measures the received signal strengths of the signals. Then, the in-vehicle device stores in advance: the vehicle-cabin inside data group containing a plurality of received signal strengths measured in a state that the portable device follows the inner side of the vehicle cabin inside-outside boundary; and the vehicle-cabin outside data group containing a plurality of received signal strengths measured in a state that the portable device follows the outer side of the vehicle cabin inside-outside boundary.
At the time of operation of the keyless entry system, the portable device measures the received signal strengths of the signals transmitted from the plurality of transmitting antennas and then transmits the measured received signal strengths to the in-vehicle device. The in-vehicle device receives the received signal strengths transmitted from the portable device and then, on the basis of the Mahalanobis distance, determines whether the received signal strengths measured by the portable device are closer to the vehicle-cabin inside data group or the vehicle-cabin outside data group. If the received signal strengths are closer to the vehicle-cabin inside data group, the in-vehicle device determines that the portable device is located in the inside of the vehicle cabin. Further, if the received signal strengths are closer to the vehicle-cabin outside data group, it is determined that the portable device is located in the outside of the vehicle cabin.
Japanese Patent No. 4673251 discloses a keyless entry apparatus having a similar configuration to Japanese Patent No. 4673234. In the keyless entry apparatus according to Japanese Patent No. 4673251, whether the portable device is located in the vicinity of the vehicle cabin inside-outside boundary is determined by using received signal strengths measured by the portable device. If the received signal strengths fall within a given interval, the portable device is determined as being located in the vicinity of the vehicle cabin inside-outside boundary and then the position determination of the portable device is performed similarly to Japanese Patent No. 4673234. If the received signal strengths go outside the given interval, the portable device is determined as being distant from the vehicle cabin inside-outside boundary and then the position determination of the portable device is performed on the basis of the magnitude of the received signal strength.